Electronic distance measuring apparatus are widely used. These apparatuses are wavelength dependant and may use signals of long waves, e.g., 150 m to 2 km wavelength, referred to as hydrographic systems for navigation purposes, microwaves, e.g., 8 mm to 10 cm wave length, and visible light of significantly shorter wavelength, e.g., in the order of about 0.6.times..sup.-6 m wave lengths. The latter referred to as optical systems often use infrared light waves generated by lasers. More commonly these systems are referred to as laser rangefinders. All of these systems are described in more detail in the literature such as Electromagnetic Distance Measurement, C. D. Burnside, 3rd Ed., BSP Professional books, London, 1991 and a book of the same title by J. M. Rueger, 3rd Ed., Springer-Verlag, New York, 1990. These texts include descriptions of apparatuses and detailed explanation of their principles of operation.
As discussed therein, higher accuracies of measurement, e.g., in optical systems operating at sub mm accuracies in atmospheric conditions that tend to vary in a way so as to cause major discrepancies in the resulting measurements. For example, in infrared or near infrared systems to maintain an accuracy better than 1 ppm, temperature needs to be known within +/-1.degree. C. Other factors that affect measurement accuracy include atmospheric turbulence, bulk refractive index of the atmosphere, pressure and humidity. For example, see the article entitled Rangefinder with Fast Multiple Range Capability, J. M. Payne et al. Rev. Sci. Instrum. 63 (6), June 1992 for further discussion on this point and which also discloses certain aspects of the invention described herein.
The present inventors are presently involved in the design of a radio telescope as also discussed in the aforementioned article. The telescope has a reflector surface which is a part of a paraboloid, so positioned that radiation can reach the reflector and then pass to one of two focal points without meeting obstructions. This clear aperture collects radiation from a circular area of 100 m diameter. The goal is to provide an instrument which performs well at radio wavelengths as short as 3 min.
To achieve satisfactory performance at short wavelengths, a radio telescope must meet two main requirements:
(1) The reflector surface must maintain its required shape. PA1 (2) The position of the telescope beam on the sky must always be controlled with precision.
These can be thought of as the "surface" and "pointing" requirements; the precision with which they must be met is related to the shortest wavelength at which the telescope is to be used, and both are dependent on atmospheric conditions at the telescope site. As an example, at a wavelength of 3 mm, it would ideally be expected to have a surface whose departures in shape from perfection had rms value of less than 0.2 mm and the astronomer would wish to point the telescope beam to within one arcsecond of any desired point in the sky.
The most significant environmental effects are due to wind and temperature. In the absence of wind, unavoidable temperature variations may result in a short-wavelength limit of .about.8 mm for a steel structure of the size of the radio telescope. As the telescope moves in elevation, many parts of the structure deform due to the force of gravity. Although these deformations can to some extent be computed, it is not easy to allow for all their effects.
The present inventors recognize a need for an accurate rangefinder system that may advantageously be used in the radio telescope to deal with the above problems. One use is to measure the shape of the reflector surface. Another use is to assist in pointing the telescope accurately in the sky. However, no known rangefinder system has the accuracy essential for these tasks, e.g., an accuracy to 50 .mu.m over a distance of about 120 meters, including considering the effects of atmospheric conditions and circuit accuracies on the resulting measurements. Further, to measure 2000 points in a radio telescope in a reasonable time frame requires a measuring rate of about 5 measurements per second. No known system can do this.